The depth, width, and flexibility of the active-site cleft of carbonic anhydrase will be probed with spin-reactivity labels. A diazene precursor molecule will be bound through an areneslfonamide residue to the enzymic active-site by complexing to the zinc atom present at the bottom. Release of a trimethylenemethane moiety by photolysis of the diazene will generate a biradical reporter group, whose rates of reaction with capture agents of various dimensions will provide information on the size of the active-site opening. Variations in the length of the molecular chain connecting the reporter group and the sulfonamide group will provide an estimate of the depth of the cleft.